Coal heated to selected temperatures in the absence of air yields coal gas, coal liquids and a residue char. Yield of the three products will vary with the temperature at which the coal is heated and the duration of time such heating is conducted. Typically such a process is facilitated by repetitively introducing batches of coal into a retort wherein the coal is heated for a period of up to 18 hours. Volatile hydrocarbon gases are released from the heated coal and are condensed into coal liquids. The remaining char is mixed with various binders and calcined to form coke for use in a blast furnace.
The batch process is usable, however, under the current economic environment, is not efficient enough to produce a satisfactory amount of coke or coal fuel to economically justify its practical application as an alternative fuel producing mechanism.
In response to this economic dilemma, various apparatus for continuously conveying coal through a retort have been devised. Conveyors and screw mechanisms are the most common. The most visible problem with these conveying apparatus is that coal assumes a plastic consistency during the pyrolyzation stage and clings to the conveyor or screw to form a residue layer that reduces the rate of conveyance of coal through the retort and eventually impairs the efficient distribution of heat through the coal being conveyed therethrough. The plasticity of heated coal also creates technical problems that hinder the continuous conversion of the char produced in the retort into an industrial metallurgical coke.
U.S. Pat. No. 1,481,627 issued to Smith discloses a method for treating coal and manufacturing briquettes wherein briquettes are calcined at approximately 1850.degree. F. Pitch is added to char to bind the briquette and to raise the percentage of volatile hydrocarbons comprised therein.
U.S Pat. No. 3,178,361 issued to Bailey discloses apparatus having a plurality of screws for facilitating the continuous carbonization of coal. The coal is heated at temperatures ranging from 500.degree. to 600.degree. F.
U.S. Pat. No. 3,251,751 issued to Lindahl et al. discloses a process for carbonizing coal using screws for conveying the coal through a retort.
U.S. Pat. No. 3,401,089 issued to Friedrich et al. discloses a process for making form coke by agglomerating discrete carbonaceous particles in a tumbling zone of a rotating retort including carbonaceous particles previously subjected to agglomeration in said tumbling zone. The process includes the steps of introducing finely divided caking bituminous coal, pitch binder and solid distillation residue of coal into the tumbling zone and calcining the mixture to form coke.
U.S. Pat. No. 3,403,989 issued to Blake et al. discloses a process for producing briquettes from calcined char, wherein the briquettes comprise 75% to 90% char.
U.S. Pat. No. 4,094,746 issued to Masciantonio et al. discloses a coal conversion process using liquefaction techniques and gas separation of coal liquids liberated by such liquefaction processing.
Exemplary of current coal conversion processes and apparatus is U.S. Pat. No. 4,395,309 issued to Esztergar. Esztergar teaches means for conveying coal over a screen encased within a retort. Heating elements above the screen pyrolyze coal causing volatile hydrocarbon to escape the retort for separation into various grades of oil.
It is very important that a chosen pyrolyzation and coke forming process produce coke that has a coke reactivity index (CRI) less than 30 and a coke strength after reaction (CRS) greater than 55. Though no official standard has been recognized to indicate at what grades coke is satisfactory for use in a blast furnace, it is recognized that coke having the aforementioned CRI and CSR ratings is high grade coke that is more than adequate for use in blast furnace operation.
CRI is determined by reacting 200 grams of 3/4".times.7/8" dry coke with carbon dioxide adjusted to a flow rate of 5 liters/minute for two hours at 1100.degree. C. (2012.degree. F.). CRI is reported as the percent weight loss of the coke sample after this reaction.
CSR is determined by tumbling the coke used during the CRI test in a drum for 600 revolutions at 20 RPMs. The cumulative percent of plus 3/8" coke after tumbling is reported as the CSR.
Variations in carbonization and calcining temperatures, residency time, briquetting pressures and the selection of binding materials mixed with the char to form coke all effect the resultant CRI and CSR of the resultant coke. Closely controlled manipulation of the coal throughout the entire procedure is a necessity for producing a high grade coke having minimal CRI and maximum CSR levels.